Clinton had sharp words for the Republican nominee, saying Trump was "dead wrong" in saying that his tax returns were not the concern of everyday Americans, despite every major presidential nominee since Richard Nixon releasing their taxes before the election.

"I think it is a fundamental issue about him in this campaign that we're going to talk about in one way or another for the next 62 days, because he clearly has something to hide," Clinton told reporters on a campaign flight to Tampa, Florida.

"If he's going to pursue this campaign, he owes it to the American people to come clean and release those tax returns."

Trump's vice presidential running mate Mike Pence, the governor of Indiana, said Sunday that he would release his own tax returns this week, but made it clear that Trump may keep his under wraps until after Election Day, November 8.

Trump has insisted that he will release his taxes, but only after the Internal Revenue Service completes its audit.

That federal agency has said Trump is free to release the returns whenever he wants. The release of such returns has been a tradition of American presidential politics for a half-century, and Democrat Hillary Clinton and her running mate Tim Kaine have already released theirs.

Trump's failure to do so has fueled speculation that he fears some embarrassing revelation: perhaps that his fortune is far smaller than the USD 10 billion he speaks of, that he has donated far less to charities than he suggests, or that he has awkwardly close business ties to Russian interests and other foreign organizations and banks.

Clinton said there is a "growing" list of activities by Trump that raise questions about his behavior and judgment, citing his companies' bankruptcies and the many lawsuits against him.

She also pointed to the ongoing investigation of his Trump University, and the accusations that his foundation donated to a group close to Florida's attorney general as the official's staff was reviewing accusations that had been made against Trump's school in the suit.

And she highlighted a New York Times report that showed that companies owned by Trump owe some USD 650 million in debt, including to foreign banks.

"The list goes on and on: the scams, the frauds, the questionable relationships, the business activities that have stiffed workers," Clinton said.

Clinton also hammered Trump on his tax plan for America, telling a voter registration rally in Tampa that Trump was peddling "trillions in tax cuts" that would "explode the national debt" and help the one percent wealthiest elite while hurting everyone else.